Support Technology And Instability Mechanism Of Large Section Coal Roadway With Weakly Cemented Soft Rock In Yangjiacun Mine

In recent years,the roadway instability phenomena with weakly cemented soft rock were frequently occurred in the western region,which had been become the important issues for many specialists,scholars and mine enterprises.Based on roof instability section of affiliated haulage gate in working face 414108 of Inner Mongolia Yanjiacun mine,the site investigation,labrotory experiment,theoretical analysis,FLAC3 D numerical simulation,engineering analogy and filed monitoring methods were utilized to prob the involing contents that physical and mechanical chracteristics of surrounding rock of weakly cemented soft rock mining roadway with large section,deforamtion traits,constitutive modeling back analysis,stability fectors and the instable type and mechanism of roof.The stability evolution law of surrouding rock was uncovered under the condtions that bury depth,height,and rise-span ratio were individually regarded as varible and that Mohr-columb Model,Ubiquitous-Joint Model,and Biliner Strain-Softening Model were respectively considered as Consitutive Model of sandy mud-stone.The "invterted wdge",high strength and high pretightening force anchor-mesh-beam-cable-guniting asymmetrical coupling technique with optimized roadway height and anchor-mesh-cable-guniting technique with optimized roadway section were proposed.The numerical simulation and site monitoring results show that the two types of optimized support techniques can be able to sustain the stability of surrounding rock,and meet the requirements of safety and high efficiency from mine enterprise.The primary conclusions are as followings:(1)The labrotory experiment,site investigation,and engineering analogy are used to attain the type of deformation and mechanics,and basic physical characteristics,and characteristics of mine pressure.The type of deformation and mechanics is ?B.The basic physical and mechanical properties of rock have relevance with water effect.The physical properties show softness,argillization,slight expansibility and stratified disintegration.When the effect is low degree,it is close to hard and brittle rock,and shows obvious brittleness and pressure-sensitivity,whereas it shows softness effect in strength and deformation indexes.Coal samples have the capacity to resist yield and failure,meanwhile the mechanical characteristics are stable.It is availbe to sustain stability of surrounding rock to utilize the better mechanical characteristics reasonably and to take advantage of bearing capacity of coal stratum.(2)The primary deformation characteristics of surrounding rock are roof-floor convergence and it is unobvious in two sides.The stability of surrounding rock in different monitoring sties has difference.The surface displacement and separation curves show “platform” and “Kick” sections,and evolution process has similar synchronism in the location of roof instability.The new characteristic is shown that deep separation value is higher than shallow monitoring point of roof.The effect factors for new trait can include following aspects: cross-section type of roadway,rock stratum characteristic,seepage and softness effect,mineral species,and initial anchor bolt technique.The multiple separation layer group of roof is the comprehensive result from seepage and softness effect,horizontal shear stress and vertical tensile stress.seepage and softness effect is the primary factor which aggravates anisotropy and deformation incompatibility of rock stratum.(3)The weakly cemented characteristics of sandy mudstone shows that the failure characteristics of rock not only include common tensile and shear failure but also show circle shear failure under the effect of the developed weak layers.The reasons of new failure type consist in two aspects: firstly,the shear and tensile strength of rock materials is deteriorated under the water effect;secondly,the shear strength of weak layers declines gradually which is the inner essence for forming weakly cemented characteristics of sandy mudstone.The characteristics of multi separation layers of roof is the superficial traits of weakly cemented characteristics.The soft rock characteristics primarily show that the strength(peak strength,residual strength,shear strength,and tensile strength)deteriorated obviously.(4)The strength characteristics of rock in the uniaxial compression are that the dilation stress threshold value is deteriorated,and the course of volume dilatancy deformation is prolonged at pre-peak.The residual stress in post-peak are obviously deteriorated.The pressure-sensitivity is unobvious,and confining pressure effect of peak and residual strength is inferior in triaxial compression.The tensile strength is deteriorated obviously.The deformation characteristics are that gradual deformation process is short,and the instable crack propagation is prolonged at pre-peak,and the deformation at post-peak has plastic flow trait and time-delay effect,and the lateral expansion is shown in the gradual failure process of rock in uniaxial compression.The water-saturated rock shows creep deformation in post-peak curve,while the dry rock shows the characteristics from brittleness to ductility in triaxial compression with confining pressure increases in triaxial compression test.The failure characteristics are that macroscopic failure of rock samples are primarily result from three types of surface including tensile fracture surface,shear slip surface,and hoop shear failure surface,and the tensile failure cracks show non-singleness.(5)The back analysis of FLAC3 D numeirical simulation is utilized to comprehensively analyze complete stress-displacement curve,peak strength,residual strength,plastic shear strain ratio,and plastic state of numerical simulation rock sample.Bilinear Strain-Softening Constitutive Model can embody complete stress-strain curves and failure traits of rock samples in laboratory reasonably,and it is reasonable and liable to study stability of surrounding rock.(6)The reasons of anchorage bolt and cable failure are that seepage and softness effect of weak aquifer leads to anchoring agent(grouted body)unstuck from wall of hole,and shear slip failure occured in the interface which results in weak suspension function and difficultly produce engineering mechanics transmission.The axial and radial force is weak,and the supporting effect of support structure becomes inferior.The reasons for gradual degradation of large and small structural body of roof are that weakly cemented characteristics of rock stratum,the multiple separation layer group,and the failure of supporting structure.The stability of roof primarily depends on "samll structure"(anchorage body beam),and the failure,gradual change,and collapse of "samll structure" is the crucial element for instability of "large structure".(7)The instable type of roof belongs to flexure failure,and the failure mechnism is that weakly cemented characteristics,multiple separation layer group,and support structure failure of rock stratum results in rock load without bearing capacity above anchorage body beam(samll structural body)increased,and anchorage body is gradually changed from clamped beam to three hinged arch.After the constant sinkage,friction,extrusion,and rotation of three hinged arch anchorage body beam,the "small structural body" is final destruction and the "large structural body " is instability.(8)When the consititutive model of sandy mud-stone is the same,the expansion and extension degree of plastic zone and the peak value of principle stress difference is roof,floor,and side with bury depth grown.The surface displacement of roadway is roof,side,and floor.The axial force of anchor bolts and cable is cable,anchor bolt of roof,anchor bolt of side.The axial force of cable and anchor bolt of roof increases gradually and is close to tensile load.The axial force of anchor bolt in the shoulder of sides depends on cable and anchor bolt of roof.When the axial force of cable and anchor bolt of roof approximates to tensile load,the axial force of anchor bolt in the shoulder of sides is enhanced with the bury depth increased,whereas the axial force decreases.(9)When the bury depth is the same,Mohr-columb Model,Ubiquitous-Joint Model,and Biliner Strain-Softening Model is respectively considered as Consitutive Model of sandy mud-stone,then the extension degree of plastic zone is roof,floor,and side.The extension degree of peak value location of principle stress difference increases gradually,and the principle stress difference of roof and side decreases gradually.The peak value of floor decreases gradually under the compressive effect of vertical stress,horizontal stress and normal stress from concrete cushion.The evolution characteristics of roof,floor,and side are accordance with extension traits of plastic zone.(10)When the consititutive model is the same,the extension degree of plastic zone of roof is the highest with the roadway height increased,and the extension degree of peak stress location of principle stress difference is roof,floor,and side.The displacement relationship between roof and side depends on consititutive model of roof.When it is Mohr-columb Model,the displacement of side is higher than roof,whereas the displacement of roof is in low degree.The axial force of anchor bolt of roof,cable,and anchor bolt of side increases gradually with the roadway increased.(11)When the roadway height is the same,sandy mud-stone stratum is respectively Mohr-columb Model,Ubiquitous-Joint Model,and Biliner Strain-Softening Model,then the extension degree of plastic zone is roof,floor,and side.The difference of peak value curves of principle stress difference is obvious.The expansion and extension degree of plastic zone,the displacement of surrounding rock,axial force of support structure is the lowest when the sandy mud-stone stratum is the Mohr-columb Model.When it is Biliner Strain-Softening Model,those are the highest,whereas those fall in between them.(12)When the rise span ratio is 1:0.1~1:0.3,the constitutive model of roof produce the weak effect for the plastic zone,principle stress difference,peak value of principle stress difference,and displacement of surrounding rock.When the rise span ratio is 1:0.4~1:0.5,the influence degree is enhanced because of constitutive and arching effect.The principle stress difference is improved,and the extension degree of plastic zone is in low degree and displacement is reduced,stability of surrounding rock is enhanced.(13)To reduce roadway height reasonably and utilize "invterted wdge",high strength and high pretightening force anchor-mesh-beam-cable-guniting asymmetrical coupling technique is vailable for expanding effective anchorage zone,enhancing the stability,strength,stiffness and bearing capacity of anchorage body structure,easing deformation incompatibility of rock stratum,playing the anchor bolt of side role to inhibit displacement of middle part of side,reduceing floor heave,deterring the transfusion of fissure water from boring hole(cable)to free face,lessening the wastage of cement in rock stratum,lowering the softening effect on roof,and enhancing the stability of surrounding rock.The self bearing capacity of roadway is enchanced,and the anti-deformation ability is improved after utilizing roadway with straight wall semicircle arch cross-section and anchor-mesh-cable-guniting support technique.The measures of keeping anchorage force should be carried out to make optimized support structure effectively bear loading in the long term.